There is known a projector in which a small-size projection unit and a direction sensor or an acceleration sensor are incorporated into a helmet (patent literatures 1, 2). This is constructed such that a facing direction of the projector is detected by the sensor incorporated in the projector and a display image is projected while being appropriately changed based on detection information.
In another projector, there is known a method for optimizing a projected image from image information by taking in an image projected from the projector again (patent literature 3). By this method, the display information of the image projected by the projector is obtained by a CCD or the like, and the display image is projected while being appropriately changed based on that information.
There is also known a method for controlling the position of a projector by providing a reference point on a screen and detecting the reference point using an image sensor (patent literature 4).
However, since the facing direction of a user is detected by the direction sensor or the acceleration sensor by the method disclosed in patent literatures 1 and 2, it is not easy to precisely detect the posture and distance of the projector with respect to a projection plane. Thus, there has been a problem of difficulty in properly projecting a display image according to the posture of the projector with respect to the projection plane and a distance of the projector to the projection plane.
On the other hand, in the construction disclosed in patent literature 3 for detecting the display image, there has been a problem of taking time to display a precise image with a mobile projector with volatile movements since it takes time for focus control and image analysis to precisely read the display image. If the display surface is on the corner or not a flat surface, e.g. with unevenness, there has been a problem of being unable to determine optimal projection conditions from image information.
By the method disclosed in patent literature 4, positioning cannot be performed unless the reference point on the screen can be detected, wherefore there has been a problem of restricting the display surface of the projector to a limited place.    Patent Literature 1:
Japanese Unexamined Patent Publication No. H07-24140    Patent Literature 2:
Japanese Unexamined Patent Publication No. H06-167687    Patent Literature 3:
Japanese Unexamined Patent Publication No. 2005-339269    Patent Literature 4:
Japanese Unexamined Patent Publication No. 2005-292563